Tractor Speed Reducer Design

ME 365 – Machine Design

Spring 2016 – Project

Tractor Speed Reducer Design

Due at 8:00am on Thursday, December 8th, 2016

Background

Design a complete single-reduction, spur gear-type speed reducer. Specify the two gears, two shafts, four

bearings, and a housing/gear box. Make detailed drawings for the two shafts (i.e. component drawings)

and an assembly drawing for the entire gear box. A tractor is being designed for chores for residential use,

such as mowing lawns. You are to design a gear pair for use in the tractor’s drive train. The pinion should

run at approximately 550 rpm. The gear should rotate at approximately 175 to 185 rpm. The wheel is 350

mm in diameter. The tractor will be driven by a 3-kW gasoline engine.

Recommendation

Use the power transmission case study presented in Chapter 18 (page 933) of your book as a guiding

example on how to design the gearbox.

Specifications

You are required to use spur gears for all the gears in the gear-box. Following are design specifications

for the gear box:

Weight and size Minimize

Shaft arrangement Parallel input and output shafts

Input Shaft: protruding 100mm out of front of gear-box.

Output Shaft: protruding 50mm out of rear of gear-box.

Provide adequate key slots on the gear box’s input shaft and output shafts.

Lubrication Some form of lubrication, such as an oil bath for splash lubrication, would be

needed for this application but you can ignore this aspect in your design.

Loading Flexible couplings may be used to connect the motor shaft to the input shaft of

the reducer and to connect the output shaft directly to the shaft of the main drive

wheel for the tractor. The design of the drive shaft has not been completed. It is

likely that its diameter will be the same as that of the output shaft of the reducer.

The loading to the input and output of the gear-box is torque only.

The desired life of the tractor is 5,000 hours of operation. The temperature range

for the tractor should be 50° F to 100° F.

Fatigue loads for bearings:

Find equivalent damage causing loads from the load case given above.

Minimum design life for bearings is 2 years.

Fatigue loads for gears:

Use 106 cycle 90% reliability data at full load conditions above.

Fatigue loads for shafts, keys, other parts:

Use infinite life for the full load output case, because of the high number

of load cycles.

Reliability and minimum factors of safety

Minimum nf = 1.5, ny = 1.4.

Select bearings for 90% reliability

Check bearings for static C0 failure using full load conditions, n = 1.2 minimum.

Design gears for 90% reliability, 106 cycles, fatigue safety factors are 1.2 surface, 1.4

bending.

Any other parts should have yield and fatigue factors of safety of at least 1.4.

Weak link design

Design the gear box’s input shaft key to fail by shearing before any of the other parts of the

gearbox yield. Failure is to occur in this weakest key first, protecting the rest of the gear-box from

accidental overloads). This will make this key the weakest link in the system. Calculate the yield

and fatigue factors of safety for this input key design.

Requirements

You are required to size all components, create an assembly drawing showing how all components fit

together (shafts, housing, bearings, gears, fasteners, etc.), and create a detailed fabrication drawing for the

intermediary shaft of the gear box. Materials for all parts to be fabricated must be called out. An example

shaft drawing is shown in the Chapter 18 of the textbook. Either SolidWorks or AutoCAD may be used.

Solid modeling is encouraged.

A stress report must be submitted that shows that all parts are adequately sized and will meet all strength

and life requirements.

Additional project requirements

Develop a computer code in Matlab, MathCAD, Maple or MS Excel, to assist you in the design process.

Please note that the design process is iterative in nature and attempting it without the use of computers is

not feasible.

Every student has to keep a logbook on the design decisions made toward the outcome of the project. The

logbook of each project team member has to be turned in with the project.

Each team member has to perform the project calculations independently and discuss their results with the

team members. After reviewing each member’s results on a given assignment, the team will submit one

optimized solution on behalf of the team. Individual member calculations have to be submitted also with

the team submission.

Penalties

Make sure to schedule your work on the project such that you meet the due dates for project deliverables

(check course schedule for due dates).

Individual member calculations have to be submitted in addition to the team submission; failure to do so

will result in a grade of zero, on that assignment, for the student who didn’t submit his/her work. No late

submissions are allowed for a team member once the team has submitted the assignment.

Failure to submit scheduled deliverables by the due date, as a team, will result in 10% grade penalty, per

day, on that deliverable (check course schedule for due dates). This clause doesn’t supersede the previous

one.

Report Outline and Grading Breakdown

Please prepare a concise and clear project report. Quality matters, not quantity.

Grading sheet

Introduction: design objective. 2 pts.

Gears design: using the given information, select gears material, and based on stress and

strength equations calculate all parameters and dimensions for gears.

Make sure that for each gear you report on: Equivalent design loads. Module. Pitch

diameter. Number of teeth. Tooth Quality (Qv). Gear face width. Gear material. Gear heat

treatment. Bending and wear factors of safety are minimized. Attachment to shaft(s).

18 pts

Summary of the gear design results 5 pts

Shaft design: analyze the load distribution and use stress-strength relation equations to

calculate shafts dimensions. Make sure that for each shaft you report on: All shafts meet

stress criteria. Factors of safety (static and fatigue). Stress concentrations. Shaft material

selection discussed. Heat treatment. Shafts fit the bearings. Shafts constrained axially. Part

drawings as needed (sizes, fillets, shoulders, key slots, etc.).

20 pts

Summary of the shafts design results 5 pts

Bearing design: select bearings according to the load, life and reliability.

Make sure that: Bearing forces are correctly resolved. Bearing sizes are consistent with

other calculations. Correct procedure were used to find Fd and C10.

12 pts

Summary of the bearing selection results 5 pts

Keys (or other locking devices): determine the shear and crushing stresses that are

being developed on the keys, and select material and size of the keys accordingly.

Calculate and /or comment on: Correct calculation method used for shear and crushing.

Slot size and depth reasonable. Factors of safety are above 2.0. Does input key protect

gearbox from overloads while adequately transmitting normal service loads?

10 pts

Summary of the key design results 5 pts

Housing: use your materials and general knowledge to determine the case dimensions,

and make sure it is durable and compact. There are no formal design calculations

required for the case.

Housing is designed so that all components can fit in, be easily assembled and

disassembled, and allow for oil to be held inside the casing for gear lubrication purposes.

5 pts

Assembly and individual drawings 10 pts

Conclusions: What did you learn in this project? 3 pts

Bonus points: Sound engineering effort made to reduce overall weight.

Innovative design (clear and reasonable). Well done drawings (see Figure 18.3, page 950

for what represents a well done drawing for the shafts and the ME180 notes for what

represents a well done drawing for the housing and assembly).

Up to

5pts in

all